If the user switches from XEC to FIRMA, they see a validation error bc 0 is not allowed. This is annoying but maybe necessary with current workflow, as bip21 requires SOME value there. So we would need to pick a default to avoid the error.
The real annoying thing though is the content jumping. We have space for the validation error but not if it takes more than 1 line.
Trying to solve this, saw a lot of these validation errors are overly long and complex. Not really useful for the user so much as dev notes.
So, tighten up the validation msgs to fix the content jumping and improve concise UX.